Add option to change battery type/chemistry to LiFePo4

[rgbduckface]

Feature request:
Add option to change battery type from the usual LiIon to LiFePo4 so the charge percentage is more exact.

Currently a fully charged LiFePo4 cell shows as 25% charged since it has a lower base voltage.

[kkwestt]

Yes! it's important to me too

[luigi311]

@rgbduckface and @kkwestt can you guys test #1176 you will need to add CELL_TYPE_LIFEPO4=1 in the build flags.